thread_pthread: do not corrupt stack

	This fixes stuck test/ruby/test_io.rb with FIBER_USE_NATIVE=0 on
	GNU/Linux because linked-list pointers used by glibc get
	corrupted when fiber stacks are copied.

	Thanks to wanabe for finding the bug and original patch.

	* thread_pthread (native_thread_init_stack): fix stack corruption
	  [ruby-core:82737] [Bug #13387]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@62712 b2dd03c8-39d4-4d8f-98ff-823fe69b080e